The present invention relates to a method of calibrating a mass spectrometer that analyzes elemental impurities in a gas stream. More particularly, the present invention is concerned with using a dry aerosol for calibrating an inductively coupled plasma mass spectrometer (ICPMS) or an inductively coupled plasma optical emission spectrometer (ICPOES).
In general, an ICPMS or an ICPOES uses an inductively coupled plasma source to dissociate the sample into its constituent atoms or ions, exciting them to a level where they emit light of a characteristic wavelength. In ICPOES, a detector measures the intensity of the emitted radiation and calculates the concentration of that particular element in the sample, whereas in ICPMS a detector measures the mass to charge ratio of the ion of that particular element and calculates the concentration of that particular element in the sample.
In principle, gases can be analyzed using ICPMS or ICPOES; however, in actuality, one can obtain only qualitative information about the gases since there are very few gas phase metal standards available. The current method for determining metallic contamination in gases is to obtain a sample in an aqueous form and analyze the aqueous solutions for metals. This is necessitated by the fact that it is easy to calibrate analytical instruments like an inductively coupled plasma mass spectrometer (ICPMS) or an inductively coupled plasma optical emission spectrometer (ICPOES), etc., using standards in an aqueous form.
In particular, the procedure currently employed by the industry consists of trapping the metallic impurities from the sample gas into an aqueous medium (e.g., hydrolyzing the gas, if the gas is hydrolyzable, or bubbling the gas through an aqueous acid to trap the metallic impurities). In most cases, the aqueous sample that is obtained undergoes a laborious sample preparation step whereby the metallic impurities trapped in the aqueous samples are transferred (using evaporation and reconstitution) to a dilute nitric acid solution. The resulting aqueous nitric acid is analyzed using either an ICPMS or an ICPOES. The sampling and sample preparation step can easily take upwards of 8 hours. The analysis is a fairly complicated process and at each step of the sampling and sample preparation there is ample opportunity for contamination which can lead to results that are higher than the actual metallic content in the sample gas. This is particularly true for elements like Na, K, Ca and Zn which are abundant in the ambient environment. Unfortunately, these elements, particularly Na and K, are critical impurities for the semiconductor industry since the presence of these elements on the layers being deposited can lead to device failures.
Ideally, one would like to introduce the gas sample directly into the analytical instrument to obtain the metallic content but, at the present time, there is no method for such a direct analysis. In particular, there have been attempts in the past to directly analyze gases using both ICPMS as well as ICPOES. Unfortunately, in all of these attempts it was recognized that calibration of the instrument is a major challenge. It was recognized that very few elemental standards can be obtained in the gas phase. In the earlier reported experiments, standards of As (as Arsine), Phosphorous (as Phosphine), Si (as Silane), Fe and Ni (as their carbonyls) and I from methyl iodide, etc. (see xe2x80x9cInvestigations into the Direct Analysis of Semiconductor Grade Gases by Inductively Coupled Plasma Mass Spectrometery,xe2x80x9d by Hutton et al., J. Anal. At. Spectrometry, Vol. 5; 1990) were used to calibrate the ICPMS. These attempts were successful because there are compounds of these elements which are gases; thus, these gases can be mixed with an inert gas (e.g., nitrogen) to form standards. This can be, in principle, extended to other elements as long as stable gas phase compounds of the other elements are available. However, a vast majority of elements, which are of concern to the semiconductor industry, are not available as gas phase compounds and no attempt was made to calibrate other elements of interest. Moreover, it was recognized that the gaseous standards used were highly toxic (including the compounds of As, P, Ni and Fe) and were a major safety concern. Consequently, most of these experiments were performed in laboratories and were able to demonstrate that gaseous samples can be directly analyzed using an ICPMS or ICPOES. Due to the issue with calibration of the ICPMS for a majority of elements that are of interest, this practice has not been carried out as a routine analytical method.
Other references related to this type of analysis are xe2x80x9cDetermination of Organometallic Compounds by Capillary Gas Chromotography-Inductively Coupled Plasma Mass Spectrometryxe2x80x9d, by Kim et al., J. of High Resolution Chromatography, Vol. 15; p. 665; 1992; and xe2x80x9cAutomated Sampling System for the Direct Determination of Trace Amounts of Heavy Metals in Gaseous Hydrogen Chloride by Atomic Absorption Spectrometryxe2x80x9d by Baaske and Telgheder, J. Anal. At. Spectrum, Vol. 10; p. 1077; 1995.
Thus, there remains a need for a method of calibrating an ICPMS or an ICPOES for analyzing impurities in directly-injected gases.
A method for calibrating an inductively-coupled plasma (ICP) spectrometer (e.g., an ICP mass spectrometer or an ICP optical emission spectrometer) to analyze metallic impurities in gases. The method comprises the steps of: (a) providing a sample gas having unknown metallic impurities therein; (b) nebulizing the sample gas with a first aqueous standard (e.g., deionized water) containing no metallic impurities into an aerosol and wherein the nebulizing comprises a known efficiency; (c) inputting and analyzing the aerosol in the ICP spectrometer to measure intensities of metallic impurities therein and wherein the analyzing comprises the application of the known efficiency to the measured intensities to form a first set of metallic impurities data; (d) conducting a standard addition process utilizing different aqueous standards of increasing concentrations of metallic impurities therein that are nebulized with the sample gas and analyzed in the ICP spectrometer to obtain a plurality of sets of data for each of the metallic impurities and to which the known efficiency is applied; (e) deriving a relationship between measured intensity and concentration for each of said metallic impurities (e.g., a calibration curve) based upon the plurality of sets of data and the first set of metallic impurities data and wherein the linear relationship defines a slope and a measured intensity intercept; and (f) determining the concentration of metallic impurities in the sample gas from the absolute value of the measured intensity intercept divided by the slope for each of the linear relationships.
A system for use in calibrating an inductively-coupled plasma (ICP) spectrometer (e.g., an ICP mass spectrometer or an ICP optical emission spectrometer) to analyze metallic impurities in gases. The system comprises: at least three different aqueous standards; a nebulizer (e.g., a microflow or a microconcentric nebulizer) for nebulizing each one of the at least three different aqueous standards in series with a sample gas having unknown metallic impurities therein to form an aerosol and wherein the nebulizer has a known efficiency; an ICP spectrometer for receiving and analyzing the intensities of metallic impurities in the aerosol to generate data regarding intensities of metallic impurities; and means for deriving a linear relationship between intensity and concentration for each of the metallic impurities based upon the data, wherein the linear relationship defines a slope and an intensity intercept, and wherein the deriving means also determines the concentration of metallic impurities in the sample gas from the absolute value of the intensity intercept divided by the slope for each of the linear relationships and wherein the deriving means takes into account the known efficiency of the nebulizer.
A system for use in calibrating an inductively-coupled plasma (ICP) spectrometer (e.g., an ICP mass spectrometer or an ICP optical emission spectrometer) to analyze metallic impurities in gases. The system comprises: at least three different aqueous standards; a nebulizer (e.g., a microflow or a microconcentric nebulizer) for nebulizing each one of the at least three different aqueous standards in series with a sample gas having unknown metallic impurities therein to form a wet aerosol; a desolvator (e.g., a membrane desolvator), coupled to the nebulizer, for converting the wet aerosol into a dry aerosol and wherein said nebulizer and desolvator have a known efficiency; an ICP spectrometer for receiving and analyzing the intensities of metallic impurities in the dry aerosol to generate data regarding intensities of metallic impurities; and means for deriving a linear relationship between intensity and concentration for each of the metallic impurities based upon the data, wherein the linear relationship defines a slope and an intensity intercept, and wherein the deriving means also determines the concentration of metallic impurities in the sample gas from the absolute value of the intensity intercept divided by the slope for each of the linear relationships and wherein the deriving means takes into account the known efficiency of the nebulizer and the desolvator.